This invention relates to a circuit breaker with overcurrent protection tripping.
An example of this type of circuit breaker is shown in Japanese Laid-Open Patent Application No. 32211/1985. It comprises a current transformer for detecting a fault current in a line conductor. The output current of the current transformer is converted into a DC signal by a fullwave rectifier and then supplied to a shunt circuit. The waveform of the signal voltage induced in the shunt circuit is the well-known absolute value waveform. The output signal of the shunt circuit is converted into a signal corresponding to its effective value or average value at a signal conversion circuit.
The output signal of the signal conversion circuit is applied to a fault processing circuit, and if the fault current is found to exceed a predetermined level, a level detection signal is input to a time delay circuit, which performs a predetermined time delay operation and triggers the gate of a thyristor to drive a release-type overcurrent trip coil and open a line contact and interrupt the current in the line conductor.
In this case, a power supply circuit for the fault processing circuit is connected in parallel with the shunt circuit.
In the above arrangement, part of the secondary current of the current transformer, which is used to detect a current, flows through the power supply circuit. As a result, the current flowing through the shunt circuit does not correspond to the current through the AC line, and an error occurs in the level of detection of the fault current. Since the current flowing into the power supply circuit is not fixed, correction of the error in the fault current level detection is difficult.
A measure for improving the accuracy of the fault current level detection is disclosed in Japanese Utility Model Application Publication No. 29931/1980. In this device, however, a capacitor forming part of the time delay circuit is associated with a leak current. In the case of a short time delay trip circuit, the leak current is very small as compared with the current corresponding to the fault current and the leak time is as short as 10msec., so the problem is not serious. In the case of long time delay trip circuit, even when the leak current is as small as 0.5.mu.A, if the current is 1.mu.A the leak current and the charging current equal and the time delay operation is twice that of the rated value. Thus it deviates from the rated value.